The top of the line of those who work for Xe are de facto CIA and & JSOC operators/operatives/officers.
There is a quirk in the budgeting for the CIA and the Pentagon that makes it significantly easier to pay someone with unique or very rare talents and skills what they are worth as a contractor. One day, someone who is helping to plan a JSOC raid is a U.S. Army solider and the next he is a Xe contractor. He still works at the same place and sits in the same seat and has the same responsibilities. Nothing has changed in who he is or how he functions, but now his pay check is bigger and it says Xe Services on it.
However, the legal ramifications are such that it's much simpler to have in-house CIA/JSOC people actually pulling the triggers. So most of what people want to throw on Xe is actually being done by the CIA/JSOC. You know, stuff like killing Bin Laden (and there were almost certainly Xe Services contractors who helped advise William McRaven and the rest of JSOC plan that operation.)
Now that budgets are being slashed and contractors have such a bad public image, expect the budgeting quirks to get ironed out, the contractors to largely disappear (well, come back in-house), and the same people to continue doing the same jobs, only with a different entity listed on their pay checks.